In many flow control applications there is a need for structures which can vary the fluid-flow rate of flowing fluids without the production of excessive wear, noise or vibration. The term "throttling" is generally applied to the function of altering or adjusting fluid flow throughout a range of flow rates. The various structures by which the function is performed are generally called "throttling valves" to distinguish them from structures whose function is to open and close a flow path as a step function. To the extent that on-off valves are not opened and closed instantaneously, so that throttling noise and vibration may be produced therein at the time of opening or closure, the invention described herein is applicable to such valves as well, and they are included in the term "throttling valve".
A typical control valve for handling the flowing of high pressure fluids employs a structure in which the cross-sectional area of the flow path is altered. This type of structure generally produces substantial noise and vibration and is quite subject to damage from cavitation. However, the structures employed in this arrangement are, as a class, least expensive and most conveniently employed. The conventional spooltype hydraulic servo valve is typical of this type of valve.
Hydraulic systems of commercial aircraft usually employ phosphate-ester-based hydraulic fluids because of their fire-resistant properties. These fluids, however, have been found to be extremely erosive in the throttling or metering control valves of these systems. In effect, they induce an electrochemical milling action on the valve metering edges which is quite apart from the normal wear associated with fluid flow. Improvements have been made in the fluids, and various attempts at valve design changes have effected some gains; however, the problem remains a severe one with valves surviving from only a very few hours to an acceptable life, but still far below that of valves that work in most other fluid systems. The phenomena is characteristic of other fluids; however, the severity with which it occurs in hydraulic systems using phosphate-ester-based fluids is particularly unique.
In systems using phosphate-ester-based fluids, one of the most erosive conditions extant can be found on valves which are underlapped (or have zero lap) and remain at null or near null for long periods of time. The configurations involved include flight control system valves, spoiler control system valves, flap control valves (which are modulating types), relief valves that have continuous low leakage or erode to that condition, and other valves that are high differential pressure-throttling configurations with continuous "built-in" or "eroded to" flow conditions. Once flow is established and the "electrochemical milling" beings, the erosion is usually continuous until the leakage rate of the valve is no longer tolerable.
Many structures have been devised in an attempt to deal with the damage resulting from operation of valves in high pressure systems. Most of these have involved some form of baffling means which operate in one way or another to divide the flow and cause the pressure drops to be taken at various locations rather than across a single metering edge. One such arrangement is described in the copending application of Ralph L. Vick, referred to above, in which flow is divided into fine streams by a series of stacked disks surrounding a spool valve and in which each small stream is caused to flow into a chamber, from thence across an orifice to another chamber, reversing direction through another orifice, etc., radially across the disks. In this arrangement the pressure drops across the disks are essentially those caused by the orifices in series. One problem which has been of some concern with this arrangement is that the orifices may become plugged or obstructed to some degree through contamination. This, of course, will degrade performance of the valve, and if one or more orifices become entirely plugged, a "dead band" will be introduced accompanied by abrupt flow increases after the spool valve land passes the dead band position. For many applications this kind of flow pattern is highly undesirable from the standpoint of control and may introduce further wear and erosion of the valve.